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which device will be slave/client and which one will be master/server

Discussion in 'Embedded' started by prasi, Nov 30, 2005.

  1. prasi

    prasi Guest

    I Have one basic doubt,I was reffering the specification of MODBUS TCP,
    I got confused.
    According to my understanding, the controlling device i.e the RTU(Real
    Time Unit) will be the slave and the one who request(poll) for the data
    is the master.
    But in case of client/server concept the one who serves the
    request(RTU) will be the server and one poll for the data is the

    Please clarify my doubt
    Thanks in advance
    prasi, Nov 30, 2005
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  2. prasi

    Thad Smith Guest

    That sounds correct.
    The master/slave allocation is a low-level communication synchronization
    concept. It says nothing about which end requests data. You may have
    one master and several slave units.

    Either master or slave may make a request of the other unit, depending
    on how the system is designed. The client/server model is better suited
    for independently running clients. A realtime system is usually more
    coordinated and doesn't fit the client/server model as well, especially
    with cental control.
    Thad Smith, Nov 30, 2005
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  3. RTU stands for "remote terminal unit". It's generally a slave
    device that provides I/O points for use by the master.

    The RTU (the remote I/O device) is the slave. In TCP terms,
    it's probably the server. The control system that reads/writes
    data to/from the RTU is the master (the client in TCP terms).
    Grant Edwards, Nov 30, 2005
  4. That's not true in older Modbus (non TCP) systems.
    Traditionally, Modbus is a single master bus. Only the master
    my make a request.

    Is Modbus TCP multi-master, or has the master-slave thing been
    thrown out completely, and it's all peer-to-peer where any device
    can send a request to any other device? In that case, all
    devices are both clients and servers in TCP terminology.
    Grant Edwards, Nov 30, 2005
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